Most electrical equipments nowadays require a power supply to receive and convert city power to DC or AC power to get a steady current for operation. The power supply may be formed in many types, such as converters, inverters and the like. One of them is flyback converter. In power supply circuits, adopting the flyback converter has many advantages, such as design is simpler and cost is lower. The circuit adopted in the flyback converter has a characteristic, namely the primary winding and the secondary winding of the transformer in the circuit to transform power have opposite polarities. When input current passes through the primary winding, the secondary winding usually does not have current passing through by employing a diode or a switch element, and energy is stored in the transformer. When the current stops passing through the primary winding, the diode is automatically set ON due to the secondary winding is induced by a potential in an opposite polarity, or the switch element is set ON to allow current to flow through the secondary winding. In order to improve efficiency, the flyback converters now on the market usually control the energy release of the secondary winding through a switch element. ON/OFF of the switch element is triggered by a control signal. Hence a special circuit is needed to control the switch element.
One of prior arts for setting off the secondary winding through a switch element is U.S. Pat. No. 7,233,505 entitled “High efficiency flyback converter with primary, secondary and tertiary windings”. It includes a primary side coupling with a primary winding (12) and a secondary side coupling with a secondary winding (22), and a tertiary winding (23) to induce polarity alteration of the secondary winding (22). ON/OFF of the secondary side is controlled by a MOSFET (25). Operation of the MOSFET (25) is controlled by an induced signal of the tertiary winding. Namely, a synchronized operation takes place on the MOSFET caused by the self-excited secondary side. Another example is U.S. Pat. No. 6,424,544 entitled “Continuous mode flyback converter”. It has a primary winding (101), a secondary winding (107) and an auxiliary winding (108). The auxiliary winding is induced polarity alteration of the primary winding to generate a signal to control operation of a transistor (110). It also sets synchronized start/stop of the secondary side through self-excited thereof. The flyback converters previously discussed that achieve synchronized control through the self-excited approach still have drawbacks. Basically, the self-excited signal is obtained through polarity alteration of the primary winding. The polarity alteration of the secondary winding results from current alteration of the primary winding. Namely, only after the polarity alteration of the primary winding has taken place, can reaction of the secondary start. Such an approach cannot achieve zero voltage switching (ZVS) effect, and a switch loss is generated. Moreover, in the event that the primary winding stops current switching due to a small load, short circuit or other abnormal conditions, the secondary winding cannot induce polarity alteration. Namely, the induced potential of the secondary winding does not change with the polarity alteration of the primary winding. Malfunction of the self-excited control mechanism occurs. And irregular and unpredictable self-excited frequency could take place due to many other factors such as load impedance, circuit impedance and the like. As a result, power waveforms generated on the secondary side become chaotic, and the flyback converter could be damaged.